Web Coverage Service (WCS)

Purpose

Summary. Coverages represent space/time varying sensor, image, simulation, and statistics data. Coverages are multi-dimensional (from 1-D timeseries to 4-D/5-D climate data) and typically represent Big Data filling Exabyte archives. A Web Coverage Service gives simple data access to coverages, a Web Coverage Processing Service allows flexible on-demand processing and filtering. However, the coverage concept as such is service-independent and can be used in any service.

Coverage service model. The OGC Web Coverage Service (WCS) standards suite defines services on coverages. The standards are available from http://www.opengeospatial.org/standards/wcs. The Core establishes basic functionality all implementations must provide: access to whole coverages or parts thereof, via spatial/temporal subsetting. Extensions add optional functionality, such as data format encodings (like GeoTIFF, JPEG2000, NetCDF), service model extensions (such as scaling and reprojection), and protocol bindings (GET, POST, SOAP, and REST).

While coverages can be served by a WCS like with any other geo service, like a Web Feature Service (WFS), Web Coverage Processing Service (WCPS), a Web Processing Service (WPS), or a Sensor Observation Service (SOS), a WCS and WCPS provide distinct, coverage-specific functionality (e.g., a WFS cannot do coverage subsetting).

Coverage data model. A coverage is a digital representation of some "space-time varying phenomenon" (loosely speaking), such as 1D sensor time series, 2D remote sensing imagery, 3D x/y/t satellite image time series and x/y/z geophysical data, as well as 4D x/y/z/t atmospheric and ocean data. OGC's normative abstract definition of coverages is given in OGC Abstract Topic 6 (which is identical to ISO 19123), OGC's normative concrete definition is provided with the OGC GML Application Schema - Coverages (nicknamed "GMLCOV"), available from http://www.opengeospatial.org/standards/gml.

Interoperability. Note that the abstract coverage definition of ISO 19123 / AT 6 does not per se ensure interoperability (many non-interoperable implementations of ISO 19123 are possible and, in fact, existing) while the concrete definition of the OGC's Unified Coverage Model, GMLCOV, is interoperable, with conformance tests down to cell ("pixel", "voxel") level starting GMLCOV 1.0 / WCS 2.0.

Implementations. A list of WCS implementations is maintained, although incomplete (OGC can only list implementations communicated, so let us know). WCS Core Reference Implementation is rasdaman.

Introductory material. Wikipedia offers general introduction and overview on coverages, Web Coverage Service, and Web Coverage Processing Service. On this OGC Networks page you find details about specification documents and files, information about supplementary material explaining use of WCS, and a list of WCS implementations & services known to us. Further, we provide an overview on the WCS Standards Working Group (WCS.SWG) agenda and some details on OGC Working Groups collaborating on coverages and WCS.

Contact points: Peter Baumann (chair) and Stephan Meissl (co-chair).

News

  • January 2013 In OWS-9, WCS has been brought close to completion by establishing a series of specifications, Abstract Test Suites (ATSs) and Executable Test Scripts (ETSs):
    • Specs added: WCS Processing Extension v2;
    • ATSs added: Range Subsetting, Scaling, Interpolation, CRS, Processing, GeoTIFF;
    • ETSs added: GMLCOV Corrigendum 1.0.1, WCS Core Corrigendum 2.0.1, CRS Extension, EO-WCS.
  • October 2012: GeoTIFF and JPEG2000 format encodings for coverage have been released by the WCS.SWG, recommending them for public Request for Comment (RFC).
  • August 2012:By vote of OGC, the WCS GET-KVP corrigendum version 1.0.1 has been accepted. It provides aslight modification necessary for the format encoding flexibility introduced with GMLCOV 1.0.1 and WCS 2.0.1.
  • August 2012:The Range Subsetting Extension has been released by the WCS.SWG, recommending it for public Request for Comment (RFC).
  • August 2012: the OGC Coordinate Reference System (CRS) Name Type Specification (NTS) [OGC 11-135] has been released by the WCS.SWG, recommending it for public Request for Comment (RFC) as a Best Practice Paper (available on this page); this document establishes a uniform naming system for CRS identifiers, allowing general multi-dimensional CRSs.
  • July 2012: Corrigenda have been published for WCS Core and GML Application Schema - Coverages (GMLCOV).
  • May 2012: rasdaman is the first WCS implementation to pass the WCS Core test suite and, therefore, becomes candidate reference implementation.
  • December 2011: a proposal for OGC Name Type Specification for Coordinate Reference Systems (CRSs) has been presented, together with an open-source registry implementation. During the next weeks this resolver will be set up as a prototype service by OGC.

History

WCS 2.0 has several significant advantages over previous versions: it relies on the GML coverage model, leading to increased interoperability as well as harmonization across OGC standards; among the main features are:

  • extends beyond pure raster grids to support all coverage types which GML 3.2.1 knows, e.g., curvilinear grids, irregular grids, point clouds, surface coverages; general meshes;
  • crisp and modular, consisting of a small mandatory core and a structured universe of optional extensions;
  • harmonized with OGC OWS-Common, GML, SWE, WMS, WCPS, and WPS
  • has a clear, formally specified syntax (XML Schema) and semantics (Schematron) easing understanding and implementation;
  • testable: for the first time, pixel contents is subject to conformance testing.

Background

"The Web Coverage Service (WCS) supports electronic retrieval of geospatial data as "coverages" – that is, digital geospatial information representing space-varying phenomena.

A WCS provides access to potentially detailed and rich sets of geospatial information, in forms that are useful for client-side rendering, multi-valued coverages, and input into scientific models and other clients. The WCS may be compared to the OGC Web Map Service (WMS) and the Web Feature Service (WFS); like them it allows clients to choose portions of a server's information holdings based on spatial constraints and other criteria.

Unlike the WMS [OGC 06-042], which portrays spatial data to return static maps (rendered as pictures by the server), the Web Coverage Service provide s available data together with their detailed descriptions; defines a rich syntax for requests against these data; and returns data with its original semantics (instead of pictures) which may be interpreted, extrapolated, etc. – and not just portrayed.

Unlike WFS [OGC 04-094], which returns discrete geospatial features, the Web Coverage Service returns coverages representing space-varying phenomena that relate a spatio-temporal domain to a (possibly multidimensional) range of properties."

-- WCS 1.1.2 [OGC 07-067r3]